With the proliferation of digital processing and signalling equipment throughout the industrial and commercial business world, there have been developed a variety of communication systems and attendant protocols for interconnecting multiple data interface sites. One of the most commonly employed links used for this purpose has been the (four-wire) telephone link, which offers the convenience of being already in place and readily extended as new business sites are developed. The exchange of messages over such a link between data/signal processing units is typically effected by way of communication modems, which interface associated digital data/signal ports with the electromagnetic transmission highway of which the telephone link is configured, the modems operating at some prescribed baud rate and interfacing messages, between the telephone link and communicating ports, that have been formatted in accordance with a prescribed communications protocol.
Now although telephone links are typically readily accessible and convenient communication highways, they are not particularly inexpensive, since usage of the links is normally priced by the number of lines employed and the distance between drops. In a business environment such as a financial institution, where a central (business) office may require the capability of communicating with a number of geographically remote branch offices, the line charges for a plurality of respective dedicated lines which interface respectively different data/signal processing, input/output equipments (e.g. automated teller machines, administrative terminals, bank security (burglar alarm) signalling units) at a main office to those at each branch office can become a significant recurring expense.
An example of a typical communication network configured in this conventional manner is illustrated in FIG. 1, which shows a main or central office 10 and a plurality N of branch or remote offices 11, 12, . . . , 1N that communicate with one another over a set (three in the example shown) of dedicated telephone lines 21, 22 and 23. Communications over telephone lines 21, 22 and 23 are carried out by a set of modems 31, 32 and 33, disposed at the main office 10, for interfacing a control processor 41, an automated teller machine controller 42 and a security controller 43, and respective modems 11-1, 11-2, 11-3; 12-1, 12-2, 12-3; . . . 1N-1, 1N-2, 1N-3 at the branch offices 11, 12, . . . 1N. These latter modems interface, in turn, the telephone lines 21, 22 and 23 with respective equipments (such as those types referenced above) 51, 52, 53; 61, 62, 63; . . . 71, 72, 73.
In the communication network of FIG. 1, when equipment (e.g. cash box controller 42) at the master site 10 wishes to communicate with equipment at a branch office (e.g. automated teller machine 62 at branch office 12), it does so by polling the remote device over its dedicated telephone link, using the protocol and baud rate prescribed for that link. Namely, communications between the central office and a remote office are conducted by providing separate dedicated modem drops along separately dedicated telephone links, and polling the individual drops from the master or central site. As mentioned previously and as will be readily appreciated from the network configuration shown in FIG. 1, as the variety of different user equipment expands to meet the particular needs of the business of interest and as additional offices are added to meet customer needs, the line charges imposed by telephone company become a major expense item and may impact the choice (and thereby the flexibility) of a multi-branch communication network. Moreover, for each newly added drop there is additional delay introduced into the operation of the network, thereby limiting the practical limits of network application.